At present, with increasingly higher requirements on power density of power devices such as converters or inverters, especially the increased requirements on power density of power modules, increasing switching frequencies of internal switches of the power modules is one effective means for reducing the sizes of the power modules. Since a power module may employ smaller magnetic components and capacitors when high frequency switches are used inside the power module, a higher power density may be obtained.
However, more switching loss will be caused by only increasing the switching frequency. FIG. 1 illustrates curves showing relationships between turn-on loss and turn-off loss and switching speeds of a switching component. In order to guarantee a smaller switching loss (including the turn-on loss and the turn-off loss) when a high frequency switch is used, the switching speed (di/dt and dv/dt) needs to be increased as shown in FIG. 1. Vce in FIG. 1 is the voltage across the two sides of the switching component, and Ic is the current flowing through the switching component. One means for increasing the switching speed is employing a relatively smaller gate driving resistor Rg. FIG. 2 illustrates curves showing relationships between the gate driving resistance and the turn-on loss and turn-off loss of the switching component. As shown in FIG. 2, taking an Insulated Gate Bipolar Transistor (IGBT) with a model number of FF400R06KE3 provided by Infineon Corporation as an example, when a gate driving resistor Rg with a smaller resistance is employed, the turn-on loss is dramatically decreased. However, if Rg is lowered, a voltage applied on the IGBT and a diode will be greatly increased. FIG. 3 illustrates curves showing relationships between the voltage applied on the IGBT and the diode and the gate driving resistance. It can be seen from FIG. 3 that, when a smaller gate driving resistance Rg is employed, there is a large voltage applied on the IGBT and the diode, especially under a large current. Thus, when a converter or an inverter is overloaded, switching components in the converter or the inverter will bear a large current when turned on or off, and if a gate driving resistor Rg with a smaller resistance is employed, a voltage spike may destroy power devices such as a converter or an inverter.
In a power module, in order to limit the voltage spike across the internal switches, various voltage clamping circuits are widely used at external lead terminals of switches in a power module. Usually, a voltage clamping circuit includes a charging loop and a discharging loop. The charging loop includes at least a clamping capacitor C and may further include a switching component connected in series with the clamping capacitor C, such as a diode D or an active switching MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and/or with a resistor. FIG. 4 illustrates a schematic diagram of charging loops of several voltage clamping circuits. FIG. 4(a) is a charging loop in an RCD voltage clamping circuit, FIG. 4(b) is a charging loop in a RC voltage snubber circuit, and FIG. 4(c) is a charging loop in a C voltage snubber circuit.
The discharging loop may include a discharging resistor R connected in series with the clamping capacitor C, i.e., the charging loop and the discharging loop share the clamping capacitor C. Further, the discharging loop may include a switching component such as a MOSFET or a diode, connected in series with the discharging resistor R.
FIG. 5 illustrates an RCD voltage clamping circuit used for an internal switch S1 when an internal circuit of a power module is a two-level and a three-level Neutral Point Clamped (NPC) type. In FIG. 5, the charging loop includes a capacitor C and a diode D connected in series with the capacitor C, and the discharging loop includes the capacitor C and a resistor R.
However, even with the voltage clamping circuit as shown in FIG. 5 connected to external lead terminals of internal switches in a power module, there is still a large actual instantaneous voltage applied on the switch S1. Taking the three-level NPC converter as shown in FIG. 5(b) as an example, according to experiments, when the current on the switch S1 being turned off is 150 A, although the instantaneous voltage of the switch S1 detected from external lead terminals is 338V, the voltage between the collector and the emitter of the switch S1 detected directly from the inside of the converter is 413V. Actually, internal switches bear a larger instantaneous voltage.